This invention relates to a process for conditioning a traffic sensor to be placed across a road transversely to to the direction of circulation of cars and trucks.
It applies more particularly to sensors that by their positioning in the road must be protected from the static or dynamic load stresses that the road undergoes. Among these sensors, the invention finds a very particular advantage for sensors requiring an installation over a considerable length, in particular the width of the road or the width of a traffic band, such as for example sensors for passage of axle assemblies.
Conventionally, sensors sensitive to the passage of axle assembles are of the piezoelectric type, so that they supply data as voltage pulses in an electric signal in correspondance with the pressure variations caused by the weight of axle assemblies at the passage of vehicles. They consist of a conductive core covered with piezoelectric ceramic in a coaxial conductive tubular shield and they are connected at one or both ends to external circuits for electric supply and treatment of the electric signals delivered.
The most frequent sensors are of the piezoelectric, capacitive, resistive or inductive type and are connected by a coaxial electric cable to a system for ensuring electric current supply and treating the outlet electric signals that reflect the measurements. This electric signal is generally delivered to one end of the sensor by means of a coaxial cable connected by its core to the core of the sensor, its shielding being at the mass or ground voltage.
Conventionally, the sensors are conditioned for their placement in the road in an aluminum U shape profile filled with a mixture of sand and of resin wherein they are embedded. The structural self-supporting bar thus obtained is then placed in a trench made in the road and also filled with a mixture of sand and resin; the sensor, its encapsulating mass and the trench extending over the width of the road.
The drawback of this technique resides in the fact that the bar containing the sensor undergoes considerable stresses, particularly shearing stresses during passage of vehicles on the road, and in the fact that it breaks under the effect of these stresses particularly because of the different moduli of elasticity that, on the one hand, the sand and resin mixture possesses while sand and resin are in approximately equal proportions and, on the other hand, the surfacing or pavement and the foundation layers of the road.
It has been proposed to attempt to absorb these stresses by providing the bar laterally with foam cover layers, but this solution is not satisfactory, since there exists the need to have a sensor encapsulated or embedded so that shows sufficient hardness in relation to the purpose of the sensor such as for example dynamic weighing, such hardness being necessary so as not to disturb the electric signal issuing from the sensor.